Eocene-Oligocene post-collisional magmatism in the Lut-Sistan region, eastern Iran: Magma genesis and tectonic implications

摘要

New ~(40)Ar/~(39)Ar, geochemical and Sr-Nd isotopic analyses of Eocene-Oligocene magmatic rocks in the Lut-Sistan region, eastern Iran shed light on their origin and the regional geodynamic evolution. Samples were collected from outcrops in the Sistan suture zone and to the west on the Lut block, which together constitute a diffuse magmatic province with an estimated size of -300 km x 400 km. Magmatism was active from the Middle Eocene (-46 Ma) to the Late Oligocene (-25 Ma). The studied rocks are dominated by volcanic and sub-volcanic rocks with minor intrusive rocks, ranging from basaltic to rhyolitic compositions with a peak in intermediate compositions. Most of them have calc-alkaline to high-K calc-alkaline affinity, although shoshonitic rocks are also present. The rocks are characterized by (i) E-MORB-like ratios involving high field strength and heavy rare earth elements, and (ii) an orogenic signature including enrichments of large ion lithophile elements and Th, Nb-Ta-Ti-P troughs and a positive Pb spike on MORB-normalized trace element variation diagrams. A juvenile mantle origin is apparent from the dominance of positive eNd(t) values for the studied rocks. The Sr- Nd isotopic compositions [(~(87)Sr/~(86)Sr)_i = 0.7042 to 0.7065; sNd(t) = - 4.9 to + 5.5] do not form parallel arrays with SiO2, Nb/La, Nb/U and Ce/Pb as would be expected for crustal contamination. Since the magmatism post-dated the Lut-Sistan collision and cannot be related directly to subduc-tion, the orogenic signature is likely inherited from the mantle source, presumably modified by sediment partial melt and fluid released from subducted slab during the Late Cretaceous Sistan subduction. We propose that the magmatism was triggered by convective removal of the lithosphere and resultant asthenospheric upwelling during extensional collapse of the east Iranian ranges in the Eocene-Oligocene.